Distributed potential charged particle detector

Abstract

A charged particle beam system for imaging and processing targets is disclosed, comprising a charged particle column, a secondary particle detector, and a secondary particle detection grid assembly between the target and detector. In one embodiment, the grid assembly comprises a multiplicity of grids, each with a separate bias voltage, wherein the electric field between the target and the grids may be adjusted using the grid voltages to optimize the spatial distribution of secondary particles reaching the detector. Since detector lifetime is determined by the total dose accumulated at the area on the detector receiving the largest dose, detector lifetime can be increased by making the dose into the detector more spatially uniform. A single resistive grid assembly with a radial voltage gradient may replace the separate grids. A multiplicity of deflector electrodes may be located between the target and grid to enhance shaping of the electric field.

I claim as follows: 
     
       1. A charged particle system, comprising:
 a charged particle column for focusing a primary charged particle beam onto the surface of a target, wherein the impact of the charged particle beam with the target induces emission of secondary particles from the target; 
 a charged particle detector assembly including:
 a detector for producing an electrical signal corresponding to the number of charged particles impacting the detector; 
 at least one grid, positioned between the charged particle detector and the surface of the target for causing charged particles to move from the target to the detector; and 
 a source of a field that deflects the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector by spreading the impinging particles more evenly over the detector, thereby prolonging the useful life of the charged particle detector. 
 
 
     
     
       2. The charged particle system of  claim 1  wherein:
 the at least one grid comprises at least two grids; and 
 the source of the field that deflects the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector comprises different potentials on the different ones of the at least two grids. 
 
     
     
       3. The charged particle system of  claim 1  wherein:
 the at least one grid comprises at least one resistive grid; and 
 the source of the field that deflects the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector comprises different potentials on the different parts of the at least one resistive grid. 
 
     
     
       4. The charged particle system of  claim 1  wherein the source of the field that deflects the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector comprises deflector electrodes positioned between the at least one grid and the target. 
     
     
       5. The charged particle system of  claim 1  wherein the source of the field that deflects the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector comprises a source of a field that deflects the secondary charged particles away from the axis of the charged particle column. 
     
     
       6. The charged particle system of  claim 1  wherein the charged particle detector comprises a multichannel plate and a collection anode; a PIN diode; or a scintillator-photomultiplier with a light optical coupling means positioned between the scintillator and the photomultiplier, configured to transmit light emitted by the scintillator into the photomultiplier. 
     
     
       7. The charged particle system of  claim 1  wherein the charged particle beam is an electron beam or a focused ion beam, and wherein the voltages on the at least one grid and the voltages on the charged particle detector are configured to collect secondary electrons. 
     
     
       8. The charged particle system of  claim 1  wherein the charged particle beam is a focused ion beam, and wherein the voltages on the at least one grid and the voltages on the charged particle detector are configured to collect secondary ions. 
     
     
       9. The charged particle system of  claim 1  wherein the charged particle beam is a focused ion beam, and wherein the source of the field that deflects the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector is configured to collect secondary ions. 
     
     
       10. The charged particle system of  claim 1  in which the source of the field that deflects the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector deflects the secondary particles in a manner that maintains the relative positions of the secondary particles from the optical axis of the column. 
     
     
       11. A method of reducing the rate of damage or contamination in a secondary particle detector in a charged particle system, comprising:
 providing a charged particle column to focus a charged particle beam onto the surface of a target, wherein the impact of the charged particle beam with the target induces the emission of secondary particles from the target; 
 providing a secondary particle detector to collect a portion of the secondary particles emitted from the target; 
 providing a field to deflect the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector by spreading the impinging particles more evenly over the detector, thereby prolonging the useful life of the charged particle detector. 
 
     
     
       12. The method of  claim 11  further comprising providing at least one grid to accelerate secondary particles from the surface toward the secondary particle detector. 
     
     
       13. The method of  claim 12 , wherein providing at least one grid includes providing a resistive grid, and further comprising a first voltage to one portion of the resistive grid and a second voltage to a second portion of the resistive grid, wherein the first and second voltages are unequal. 
     
     
       14. The method of  claim 12  wherein:
 providing at least one grid includes providing two grids; and 
 providing a field to deflect the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector comprises providing different potentials on the different ones of the at least two grids. 
 
     
     
       15. The method of  claim 12  wherein:
 providing at least one grid includes providing at least one resistive grid; and 
 providing a field to deflect the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector comprises providing different potentials on the different parts of the at least one resistive grid. 
 
     
     
       16. The method of  claim 12  wherein providing a field to deflect the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector comprises providing deflector electrodes positioned between the at least one grid and the target. 
     
     
       17. The method of  claim 11  further in which providing a secondary particle detector includes providing a secondary particle detector between the column and the target. 
     
     
       18. The method of  claim 11  wherein providing a field to deflect the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector comprises providing a field that deflects the secondary particles away from the axis of the charged particle column. 
     
     
       19. The method of  claim 11  wherein providing a secondary charged particle detector comprises providing a multichannel plate and a collection anode; a PIN diode; or a scintillator-photomultiplier with a light optical coupling means positioned between the scintillator and the photomultiplier, configured to transmit light emitted by the scintillator into the photomultiplier. 
     
     
       20. The method of  claim 11  wherein providing a charged particle column includes providing an electron beam or a focused ion beam, and wherein the secondary particle collector collects secondary electrons. 
     
     
       21. The method of  claim 11  wherein providing a charged particle column includes providing a focused ion beam, and wherein the secondary particle collector collects secondary ions. 
     
     
       22. A charged particle system, comprising:
 a charged particle column for focusing a primary charged particle beam onto the surface of a target; 
 a charged particle detector assembly including:
 a detector for producing an electrical signal corresponding to the number of charged particles impacting the detector; 
 at least one grid, positioned between the charged particle detector and the surface of the target for causing charged particles to move from the target to the detector; 
 a source of a field that deflects the secondary charged particles to reduce the maximum current density of the charged particles impinging on the charged particle detector by spreading the impinging particles more evenly over the detector, thereby prolonging the useful life of the charged particle detector.